-The stupid romance shit that never went anywhere (also see Regular Show)
-The stupid building up to stories that never went anywhere, seriously
-Dedicating far too many episodes not focused on the main characters
-Trying too hard to make the show into a drama and changing characters like Ice King, Marceline, Bubblegum, etc too much
-Making too many badly paced episodes that felt rushed or unsatisfying
Those are the main points. Seasons 1, 2, 3 are good, but the rest should be approached in a more insightful manner. There are some good episodes, but the quality does drop.
